The present invention relates generally to electrical test probes and more particularly to an apparatus for positioning a hand-held electrical test probe on leads of an integrated circuit device mounted on a substrate.
Integrated circuit packages have evolved from the early days of dual in-line packages (DIP) having a limited number of leads with 100 mil spacing between the leads to today's surface mounted packages having tens or hundreds of leads with lead spacings of 50, 25, and 19.6 mils. In addition, package shapes are no longer confined to the DIP configuration. For example, microprocessor chips and ASIC devices come in a variety of rectangular shapes and sizes. Hand held probing of these fine lead pitch devices is very difficult. It is very easy to short two adjacent leads together with the probe when trying to make a measurement on a single lead.
U.S. Pat. No. 4,949,032 describes a probe for dual in-line packages that has a detachable gripping portion that includes an extensible claw for gripping the DIP on one side and dual prongs having an extended contact disposed therebetween for connecting to a lead to be probed on the other side. The extended contact is a fairly stiff gold plated rod that is connected to a contact stem having an exposed center conductor. The center conductor is surrounded by an insulating material which in turn is surrounded by an outer shield of stainless steel miniature tubing. The center conductor and the steel tubing are respectively connected to the center conductor and the outer shielding conductor of a coaxial cable connected to a measurement test instrument. The detachable portion also includes a finger grip and a thumb grip by which the thumb and forefinger of one hand can cause the extensible claw to move away from the prongs. A compression spring causes retraction of the extensible claw when the pressure of the thumb and forefinger is released, thus gripping the DIP between the claws and the prongs.
One of the drawbacks to the above described gripping device is that it is specifically designed for DIP devices. The extensible claw would have to be considerably lengthened in order to grip current surface mounted microprocessor and ASIC packages. This would require extending the length of the gripper, which in turn would require lengthening the contact stem. This, in turn, would further increase the inductive and capacitive loading on the lead being probed. The probe is described and shown as being directly connected to a coaxial cable without any intervening compensation circuitry. This configuration severely limits the bandwidth of the probe. To faithfully reproduce signals from a device under test over a wide range of frequencies, some form of compensation circuitry needs to be included in the probe head. The circuitry may be passive or active and is designed to minimize the capacitive and inductive loading at the probe tip. The lack of any compensation in the probe is a further drawback to this design.
U.S. Pat. No. 4,767,985 describes another type device for use on IC packages. The device has a unitary body having an overall shape and configuration that allow it to fit over the IC package. The unitary body has comb teeth on one end for orienting the body over the leads of the IC. The unitary body also has gripping means having hook end for securely fastening the probe device to the IC device. A sliding plate means having holes formed therein that are aligned with the leads of the IC is provided inward lateral pressure on the gripping means for causing the hook ends to grip underneath the IC device. The unitary body also has at least one threaded screw hole to allow a size 4-40 machine screw to pass therethrough to provide downward pressure on the sliding plate to maintain the hook ends in gripping contact with the IC device. The holes in the sliding plate accept spring loaded electrical contact probes for making contact with the leads of the IC device.
Similar types of adapters for surface mounted IC device are described in U.S. Pat. No. 5,015,946 and U.S. Pat. No. 5,202,622. Both patent describe adapters for probing surface mounted integrated circuit devices with the first referenced patent having a housing that has a skirt with alignment surfaces for mating with the IC package to provide initial alignment. An alignment plate is elastically mounted within the probe housing internal of the skirt. The alignment plate has serrations between pin holes with the pin holes corresponding to probe pins elastically mounted on the probe housing in a pattern matching the leads of the integrated circuit package. When the probe is pressed down over the integrated circuit package the alignment surfaces within the skirt align the probe to the package, and the serrations mesh with the leads to align the probe pins with the leads, the probe pins extending through the pin holes and maintaining positive contact due to the elastic mounting of the pins. A base plate having a central aperture with comblike teeth extending into the aperture may first be placed over the integrated circuit package with the teeth meshing with the leads to provide a surface to mate with the alignment surface within the skirt. The probe housing also has an integral connector section that provides access to the probe pins. The connector section may have a plurality of individual connectors or a plurality of flexible cable outlets, both of which are connected to a measurement test instrument, such as an oscilloscope, logic analyzer or the like.
The other referenced patent describes an test fixture for a surface mounted IC device having a test head assembly connected to an adapter having electrically conductive elements that couple the electrical signals on each of the leads of the IC device to individual test points on the test head assembly. The adapter has a housing positionable on the IC device with the electrically conductive runs being disposed in the housing and held in place by coverplates. Each electrically conductive element has one end that engages a lead on the IC device and another end that forms an electrical contact pad that is exposed on the top of the housing. The test head assembly has corresponding contact pads that mate with the contact pads of the electrically conductive elements. The test head assembly contact pads are electrically connected to corresponding test points.
The three above described devices have some common characteristics. They all have a structure or unitary body that is device dependent. That is the housing or unitary body is specifically designed to fit over a particular IC device having a particular size and shape. To accommodate the various sizes and shapes of today's IC packages requires a multitude of devices having dimensional characteristics that meet each particular IC package shape. Further, the unitary body design uses spring loaded electrical contact probes, such as the P2662A Series Probes along with sockets 52662A that are manufactured and sold by Pylon Company, Inc. These pogo pin type probes suffer from the same problem as the contact stem of the '032 patent in that there is no compensation circuitry in the probing tip. While the '946 patent describes the use of individual probing tips within the housing for each lead of the IC device, it does not address the problem of providing compensation close to the probe tip. The '622 patents allows the use of hand held electrical test probes for probing test points of the test head assembly. However, there is still an extended signal path between the leads of the IC and the test points.
What is needed is an apparatus that allows direct hand-held probing of individual leads of a surface mounted integrated circuit device having fine lead pitch spacing using a hand held electrical test probe. The apparatus should be simple and inexpensive to manufacture and use and should not be constrained by the packaging shape or size of the IC device.